Integrated high frequency balanced-to-unbalanced transformers

ABSTRACT

Integrated high frequency balanced-to-unbalanced transformers suitable for operation in high frequencies, such as radio frequencies. Embodiments disclosed give consideration to issues related to the layout of the primary and secondary inductors for the minimization of capacitive effects between layers while using a minimal number of metal layers. Two solutions are provided, one having embodiments with a symmetrical primary inductor in a 4-metal layer implementation and one having embodiments with a non-symmetrical primary inductor in a 3-metal layer implementation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 60/605,511 filed Aug. 31, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor integrated circuits, andmore particularly to integrated circuit inductors that aremagnetically-coupled for the purpose of creating a high frequencytransformer.

2. Prior Art

In wireless communications, an antenna is commonly coupled, typically bymeans of passive components, to a transformer. In many cases, abalanced-to-unbalanced (BALUN) transformer is used. Such a transformerallows the conversion of a single-ended signal into a differentialsignal and vice versa. In wireless communications, the antenna receivesa single-ended radio frequency (RF) signal. The signal is converted to adifferential signal using a BALUN transformer. The operation of BALUNtransformers is well-known in the art, and such transformers are usuallyrepresented by the schematic 10 shown in FIG. 1A. The unbalanced side ofthe BALUN transformer has two ends marked 12 and 14 respectively. Thebalanced side of the BALUN transformer has three connections, two oneach end of the inductor marked 22 and 26 respectively, and one at thecenter of the inductor, marked 24. The balanced side provides for adifferential mode. In some cases the inductors of the transformer arewound around a core, directly impacting the mutual inductance betweenthe primary and secondary inductors and therefore the performance of thetransformer. Typically node 14 of the primary inductor and node 24 ofthe secondary inductor are AC grounded, as shown in FIG. 1B.

With the advent of solid state electronics, the ability to integratecomponents in a single semiconductor device has increased manyfold. Thisallows the reduction in size, power consumption, and cost, and furtherprovides overall improvement in system performance. It is thereforenatural that many attempts have been made to integrate transformers,including BALUN transformers, in order to take advantage of thesefeatures. Providing a symmetrical BALUN transformer has been known to bea challenge in the art, as specifically shown in U.S. Pat. No. 6,608,364by Carpentier (hereinafter “Carpentier”) and U.S. Pat. No. 6,707,367 byCastaneda al. (hereinafter “Castaneda”). Carpentier suggests animplementation of a BALUN transformer that has five metallizationlayers, therefore requiring a complex manufacturing process having manylayers often restricting conductor routing over the BALUN transformer.Castaneda suggests an elaborate scheme to provide a symmetrical BALUNtransformer, also requiring several layers of metal and dielectric asshown in the various Figs. of Castaneda. Another example may be found inU.S. Pat. No. 6,882,263 by Yang et al. Symmetrical primary and secondarywindings of an on-chip BALUN transformer are shown. However, the issueof capacitive coupling between the primary and secondary windings is notaddressed, as the windings are essentially positioned such that amaximum capacitive coupling is achieved, having a disadvantage inoperation at high frequencies, for example several GHz, as thecapacitive coupling will tend to short-circuit the BALUN at these higherfrequencies.

As the demand for integrated circuit radios increases, many attemptshave been made to integrate transformers and/or transformer BALUNs ontoradio frequency integrated circuits. However, such integration has beenlimited due to flux leakage, capacitive coupling limits, and significantseries resistance. To reduce these limitations, advances have been madein transformer IC design including coplanar interleaved transformers,toroidal and concentric transformers, overlay transformers and symmetriccoplanar transformers. Coplanar interleaved transformers have theprimary and secondary windings interleaved on the same integratedcircuit layer, where the primary and secondary windings are constructedof planer metal traces. While coplanar interleaved transformers reducesize and are widely used, they suffer from low quality (Q) factor, smallcoupling coefficients, and, if used as a BALUN, the center tap is oftenat an undesirable location, resulting in an asymmetric geometry. As isknown, asymmetry of a transformer winding causes an imbalance in theresulting differential signal and/or an imbalance in the resultingsingle ended signal from a differential signal.

There is therefore a need in the art for a BALUN transformer which isessentially symmetrical, can be implemented in a minimal number oflayers of metal, and still provide the electrical characteristics of aBALUN transformer, and especially a reduced capacitive coupling, for thepurposes of RF applications, for example in the gigahertz range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic drawing of a BALUN transformer.

FIG. 1B is a schematic drawing of a BALUN transformer with a groundednode of the primary coil and a grounded center node of secondary coil.

FIG. 2 is an exemplary layout of a primary inductor of a first BALUNtransformer using a single metal layer.

FIG. 3 is an exemplary layout of a symmetrical secondary inductor of thefirst BALUN transformer having a displacement with respect to theprimary inductor.

FIG. 4 is an exemplary layout of the shunts of the first BALUNtransformer.

FIG. 5 is a layout of the first BALUN transformer showing therelationship between all three metal layers.

FIG. 6 is an exemplary layout of a pseudo-differential and symmetricalprimary inductor of a second embodiment of BALUN transformer.

FIG. 7 is an exemplary layout of a symmetrical secondary inductor of thesecond BALUN transformer having a displacement in respect to the primaryinductor.

FIG. 8 is an exemplary layout of the shunts for the primary inductor ofthe second BALUN transformer.

FIG. 9 is an exemplary layout of the shunts for the secondary inductorof the second BALUN transformer.

FIG. 10 is a layout of the second BALUN transformer showing therelationship between all four metal layers.

FIG. 11 is a diagram of a portion of primary coil metal layer and thesecondary coil metal layer showing the displacement between therespective windings.

FIG. 12 is a flowchart of the process steps respective of themanufacture of the BALUN transforms in accordance with the disclosedinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Deficiencies of the prior art have lead to a need to provide BALUNtransformers that are more efficient in their design, particularly inthe number of metallization layers used for their implementation withoutsignificantly adversely affecting the BALUN transformer performance. Thesolution of the present invention accomplishes this target by having thewindings of the primary inductor in one metal layer and the windings ofthe secondary inductors in another metal layer not only verticallyseparated from, but also horizontally displaced from the first metallayer. The displacement reduces the capacitive coupling between theprimary and secondary coils. Furthermore, the implementations shownenable the use of only three or four layers of metal for a BALUNtransformer. It should be noted that the displacement should be suchthat a substantial magnetic coupling between the primary and secondaryinductors of the BALUN is still achieved to ensure the properperformance of the BALUN.

Reference is now made to FIGS. 2 through 4 where each of the three metallayers comprising a BALUN transformer 500, shown in FIG. 10, areimplemented in accordance with the disclosed invention are shown. Theimplementation makes use of three metal layers, metal layer 100, metallayer 200, and metal layer 300. A person skilled-in-the-art will realizethat it is not required that the metal layers used are consecutive metallayers, and specific choices may be made for the desired characteristicsof the BALUN transformer, such as BALUN transformer 10, including, butnot limited to, the grounding of both one of the nodes, for example node14, of the primary inductor and the center node 24 of the secondaryinductor.

In FIG. 2, a primary coil is composed of a continuous winding 210 andends 12 and 14, implemented on a metal layer 100, and designed to bepseudo-symmetrical, with a slight asymmetry when curving to implement aninternal winding. In FIG. 3 a secondary coil, implemented in metal layer300, is composed of winding segment 310 having ends 312 and 26, windingsegment 320 having ends 22 and 322, and winding segment 330 having ends332 and 334. The complete coil of the secondary coil is achieved by theuse of shunt 410, connecting ends 322 and 334 of winding segments 320and 330 respectively, and shunt 420, connecting ends 312 and 332 ofwinding segments 310 and 330 respectively. The shunts are shown in FIG.4.

Winding segments 310, 320 and 330 of the secondary coil of FIG. 3 have adisplacement with respect to winding 110 of the primary coil of FIG. 2,as explained in more detail below. The displacement reduces thehorizontal overlap between the primary and secondary coils and hencereduces the capacitive coupling between them. Preferably thedisplacement is such that there is less than fifty percent overlap inthe conductive paths between the windings of the secondary and theprimary windings, excluding the shunts. A non-overlapping implementationis also possible as long as there is sufficient magnetic couplingbetween the primary and secondary inductors of the BALUN. In someembodiments of the disclosed invention, the input nodes of the primaryinductor are physically one-hundred and eighty degrees from the outputsof the secondary inductor, further achieving symmetry. FIG. 5 shows suchan embodiment, with the center tap 24 of the secondary being connectedto node 14, typically both being grounded or coupled to a circuit commonby a single connection thereto.

Referring now specifically to FIG. 5, a top view of the three metallayers comprising BALUN transformer 10 are shown. In one preferredembodiment, metal layer 100 is the bottom layer, metal layer 200 is themiddle layer and metal layer 300 is the upper layer. In particular, theprimary coil metal layer 100 would be deposited over an insulator suchas silicon dioxide (SiO₂), for example on a substrate, typically asilicon substrate, and then patterned using conventionalphotolithography techniques. Notably, metal layer 100 may be any one ofthe metal layers available for use in the device. Then another SiO₂layer is deposited, followed by the depositing and patterned of anothermetal layer 200 to form the shunts. A further SiO₂ is deposited andwindows opened (etched) therein to expose the ends of the shunts forVIAs, and in the embodiment being described, an opening through the lasttwo SiO₂ layers to expose node 14 of the primary inductor. Then a finalmetal layer is deposited and patterned, making electrical contact withthe shunts the form the complete secondary winding, and providing acommon connection to one primary node (14) and the center node 24 of thesecondary winding. It should be further noted that it is not requiredthat the metal layers, used in the BALUNs of the present invention, beconsecutive metal layers. Hence if a semiconductor device has availablea total of seven metal layers, then if three metal layers are used forthe BALUN, any three of the seven metal layers may be of use.

By using this arrangement, the vertical distance between the primarycoil and the secondary coil is further increased and thereforecontributes to a reduction in the capacitive coupling between the coils.The primary coil is accessed at nodes 12 and 14 in metal layer 100.Since node 14 is connected to the center node 24 of the secondaryinductor, it is further possible to access node 14 in metal layer 200.The secondary coil ends 22 and 26 are accessed in metal layer 300, whilecenter node 24 of the secondary coil is accessed at end 24 in metallayer 300, as well as through node 14 in metal layer 100 as explainedabove.

In one alternate embodiment, the order of the layers may be reversed,namely layer 300, then layer 200 and finally layer 199. In anotherembodiment of the disclosed invention, metal layer 300 follows metallayer 100 in the vertical stack, with the last metal layer being metallayer 200. Connection between layers is achieved by the use of VIAs orstacked VIA holes which are well-known in the art. The inventors havenoted that this implementation provides for minimal losses and has anarrowband balancing.

Typical external diameter for a BALUN transformer in accordance with thedisclosed invention is between 200 and 800 micron. Spacing between turnsin the primary coil is typically 5 to 10 microns, and between turns ofthe secondary coil is typically 5 microns. A conduction path width ofthe primary inductor is typically between 10 and 20 microns and thesecondary inductor path width is typically 5 microns. Therefore, in apreferred embodiment of the invention, with a fifty percent overlap ofthe secondary with respect to the primary, only 2.5 micron of width, orless, of the secondary inductor will be in overlap with the windings ofthe primary inductor. The typical numbers provided herein are of courseexemplary only, and are not intended to limit the scope of the disclosedinvention.

Reference is now made to FIGS. 6 through 9 where each of the four metallayers comprising a BALUN transformer 1000, shown in FIG. 10 inaccordance with another embodiment of the present invention are shown.This embodiment is designed to provide broadband balancing. Theimplementation makes use of four metal layers, metal layer 100, metallayer 200, metal layer 300, and metal layer 400. These layers are shownin FIGS. 6 through 9. A person skilled-in-the-art will realize that itis not required that the metal layers used be consecutive metal layers,and specific choices may be made to accommodate the specificcharacteristics of BALUN transformer 1000. The schematic of BALUNtransformer 1000 is identical to the schematic shown for BALUNtransformer 10 in FIG. 1B, and therefore node designation shall againremain the same.

In FIG. 6, a primary coil is composed of a winding segment 610 havingends 12 and 612, and a winding segment 620 having ends 622 and 14.Winding segments 610 and 620 are implemented in a patterned metal layer100. In FIG. 8, there is shown a shunt 810 implemented in patternedmetal layer 200. Shunt 810 connects ends 612 and 622 of windings 610 and620 respectively. By connecting winding segments 610 and 620, shunt 810completes an implementation of a primary coil of BALUN transformer 1000,creating a pseudo-differential inductor, having only two spirals. InFIG. 7 a secondary coil is composed of winding segment 710 having ends22 and 712, winding segment 720 having ends 26 and 722, and windingsegment 730 having ends 732 and 734. Segments 710, 720 and 730 of thesecondary coil of BALUN transformer 1000 are implemented in patternedmetal layer 400. In FIG. 9 there are shown shunts 910 and 920implemented in patterned metal layer 300. Shunt 910 connects ends 722and 734 of windings 720 and 730, and shunt 920 connects ends 712 and 732of windings 710 and 730. By connecting winding segments 710, 720 and730, shunts 910 and 920 complete an implantation of a differentialsecondary coil of BALUN transformer 1000, where typically center node 24is grounded, and connected to one of the nodes of the primary coil, forexample node 14. Winding segments 710, 720 and 730 have a displacementwith respect to winding segments 610 and 620 of the primary coil, asexplained in more detail below. The displacement reduces the overlapbetween the primary and secondary coils and hence the capacitivecoupling between them. Preferably the displacement is such that there isless than fifty percent overlap in conductive path width between thewindings of the secondary and the primary windings, excluding theshunts. A non-overlapping implementation is also possible as long asthere is sufficient magnetic coupling between the primary and secondaryinductors of the BALUN. In one embodiment of the disclosed invention,the output nodes of the primary inductor are physically one-hundred andeighty degrees from the outputs of the secondary inductor, furtherallowing for achieving symmetry.

Referring now to FIG. 10, the four metal layers comprising BALUNtransformer 1000 are shown. In one preferred embodiment, metal layer 100is the bottom layer, metal layer 200 is a first middle layer followed bymetal layer 300, and metal layer 400 is the upper layer. However, aperson skilled-in-the-art would easily note that a reverse order couldbe used, or in fact, any order that would not cause a restriction on theconnection between the different metal layers. The primary coil isaccessed at ends 12 and 14 in metal layer 100. End 14 may be furtheraccessed via node 24 of the secondary coil, connected through shunt 24shown in FIG. 8. The secondary coil ends 22 and 26 are accessed in metallayer 400. Center node 24 of the secondary coil is accessed via metallayer 200 which is also connected, for example by use of a VIA to node14 in metal layer 100. Connection between layers is achieved by the useof VIA or stacked VIA holes which are well-known in the art. Thefabrication process in general may be similar to that previouslydescribed.

In the BALUNs of the present invention, each layer is separated fromadjacent layers by an electrically insulative (dielectric) layer,preferably SiO₂, though other substrates and other electricallyinsulative layers could be used if desired. In that regard, silicon andSiO₂ are preferred as being most compatible with integrated circuitfabrication processes. The metal layers may be of various metals, thoughhigh electrical conductivity metals are preferred, such as aluminum,gold or silver. It should be further noted that it is not required thatthe metal layers, used in the BALUNs of the present invention, beconsecutive metal layers. Hence if a semiconductor device has availablea total of seven metal layers, then if three metal layers are used forthe BALUN, any three of the seven metal layers may be of use.

The inventors have noted that the foregoing implementation provides forminimal losses and has a broadband balancing. Typical external diameterfor a BALUN transformer in accordance with the disclosed invention isbetween 200 and 800 micron. Spacing between winds in the primary coilare typically 5 to 10 microns, and between windings of the secondarycoil are typically 5 microns. A path width of the primary inductor istypically between 10 and 20 microns and the secondary inductor istypically 5 microns. Therefore, in a preferred embodiment of theinvention, with a fifty percent overlap, only 2.5 micron of width, orless, of the secondary inductor conductive path will be in overlap withthe windings of the primary inductor. Again, the typical numbersprovided herein are exemplary purposes only and are not intended tolimit the scope of the disclosed invention.

Reference is now made to FIG. 11 where a diagram of a first portion 1110of a primary coil metal layer and a second portion 1120-A and a thirdportion 1120-B of a secondary coil metal layer are shown. The layout ofthe second portion and third portion is in displacement with respect tothe first portion. By avoiding full coverage between the primary andsecondary coils, the parasitic coupling capacity is reduced and overallperformance of the BALUN transformer improved. This separation furtherallows the use of a wider first portion and therefore reduces theresistance of the primary inductor.

Reference is now made to FIG. 12 where an exemplary flowchart 1200 ofthe process of manufacture of the BALUN transformers disclosed herein isshown. In one embodiment of the manufacturing process, in step S1210Athere is created in a first metal layer an essentiallypseudo-symmetrical winding. Alternatively, step S1210B is used wherethere is created a first winding that is symmetrical, as explained abovewith respect to FIG. 6. In step S1220 there is deposited a layer ofnon-conducting material that is an insulator between one layer of metaland another layer of metal, and has further known dielectriccharacteristics. Therefore, when depositing another metal plate on topof the dielectric, there will be formed a parasitic capacitor, knownalso as a coupling capacitance, between the two layers of metal,reducing the performance of the BALUN. In accordance with the disclosedinvention, in step S1230 there is created a symmetrical second winding,as may be seen with respect to FIGS. 3 and 7, the second winding beingconcentric with, but horizontally displaced from the turns of the firstwinding. In one embodiment, the overlap between the second winding andthe first winding is no more than fifty percent of the conductive pathwidth of the second winding, excluding shunts. A non-overlappingimplementation is also possible as long as there is sufficient magneticcoupling between the primary and secondary inductors of the BALUN. Insome embodiments of the disclosed invention, the output nodes of theprimary inductor are physically one-hundred and eighty degrees from theoutputs of the secondary inductor, further providing symmetry. In stepS1240, shunts are created to ensure continuous conducting paths througheach of the first winding and the second winding. A person skilled inthe art would readily recognize that the shunts may be created atmultiple metal layers and hence the specific order shown herein shouldnot be viewed as a limitation of the invention. Furthermore, it shouldbe noted that the preferred manufacturing processes in general arewell-known in the art, and are not provided herein in great detail inorder to maintain clarity of the disclosed invention. Also while certainpreferred embodiments of the present invention have been disclosed anddescribed herein for purposes of illustration and not for purposes oflimitation, it will be understood by those skilled in the art thatvarious changes in form and detail may be made therein without departingfrom the spirit and scope of the invention. As an example, while inembodiments shown herein with respect of FIGS. 6, 7 and 8, where theprimary inductor has two turns and the secondary inductor has threeturns, other configurations may be used. For example, and withoutlimitation to the disclosed invention, embodiments of a BALUN havingthree turns in the primary inductor and five turns in the secondaryinductor, or, four turns in the primary inductor and seven turns in thesecondary inductor, are also possible.

1. An on-chip three-metal layer balanced-to-unbalanced (BALUN)transformer comprising: a first winding conductive path formed in afirst metal layer defining a multiple turn primary inductor of the BALUNtransformer, the first winding conductive path having a first port atone end of the first winding conductive path and a second port at theother end of the first winding conductive path; and, a second windingconductive path having a plurality of symmetrical winding segmentsformed in a second metal layer; shunts formed in a third metal layer andelectrically connecting the winding segments at respectiveintersections, thereby forming a secondary inductor of the BALUNtransformer, the secondary inductor having a third port and a fourthport and further having a center tap connection from a positionessentially at the middle of the secondary inductor as a fifth port; thesecondary inductor conductive path being displaced vertically andhorizontally from the first winding conductive path to reduce capacitivecoupling between the primary inductor and the secondary inductor whilemaintaining a sufficient magnetic coupling between the first windingconductive path and the secondary inductor.
 2. The BALUN of claim 1wherein the primary inductor comprises a plurality of concentricwindings, ends of adjacent concentric windings being electricallyconnected by an integral metal segment in the first metal layer.
 3. TheBALUN of claim 1 wherein the ports of the first winding conductive pathare physically positioned one-hundred and eighty degrees from the portsof the secondary inductor.
 4. The BALUN of claim 1 wherein the secondwinding conductive path has an overlap with the first winding conductivepath not exceeding fifty percent of a width of the second windingconductive path.
 5. The BALUN of claim 1 wherein the displacementresults in no overlap between the first winding conductive path and thesecond winding conductive path.
 6. The BALUN of claim 1 wherein thelayers are physically ordered first metal layer, third metal layer andthen the second metal layer.
 7. The BALUN of claim 6 wherein the orderstated is with the first metal layer closest to a substrate on which theBALUN is formed.
 8. The BALUN of claim 1 wherein the ports of the firstwinding conductive path are physically positioned one-hundred and eightydegrees from the third and fourth ports of the secondary inductor, andwherein the center tap of the secondary inductor is in electricalcontact with the one of the first and second ports of the primaryinductor.
 9. An on-chip balanced-to-unbalanced (BALUN) transformercomprising: a first winding conductive path formed in a first metallayer defining a multiple turn primary inductor of the BALUNtransformer, the first winding conductive path having a first port atone end of the first winding conductive path and a second port at theother end of the first winding conductive path; and, a second windingconductive path having a plurality of symmetrical winding segmentsformed in a second metal layer defining a secondary inductor of theBALUN transformer, the secondary inductor having a third port and afourth port and further having a center tap connection from a positionessentially at the middle of the secondary inductor as a fifth port; thesecondary inductor conductive path being vertically and horizontallydisplaced from the first winding conductive path to reduce capacitivecoupling between the primary inductor and the secondary inductor whilemaintaining a sufficient magnetic coupling between the first windingconductive path and the secondary inductor.
 10. The BALUN transformer ofclaim 9, wherein the first winding conductive path is symmetrical.